1. Field of the Invention
This invention relates generally to high-speed, low power digital integrated circuits. More particularly, this invention relates to high-speed, low power digital integrated circuits for complementary gallium arsenide fabrication processes.
2. Description of the Prior Art
The demand for high-speed, low-power, digital integrated circuits in computers and other digital systems for spacecraft, aeronautical, mobile, portable, and weapons applications is rapidly increasing. Existing circuits provide either high performance at high power consumption or low power consumption but low performance.
Gallium arsenide (GaAs) is a compound semiconductor that can be used to fabricate field effect transistors (FETs). Like silicon (Si) FETs, GaAs FETs can be used to make both analog and digital integrated circuits (ICs). GaAs FETs have been in use in analog microwave circuits since about the late 1960s, where they provide bandwidth, noise, and power consumption advantages over available silicon devices for certain applications. GaAs FETs have been in use in digital integrated circuits since about 1974. Initially, their use was limited to extremely high-speed applications where logic density was not a major issue and where power consumption was less of a concern than operating speed.
Today, the use of GaAs FET digital ICs is very common in high-performance digital systems. The speed of GaAs FET logic has surpassed the speed of Si Complementary Metal Oxide Semiconductor (CMOS) logic, Si Bipolar Complementary Metal Oxide Semiconductor (BiCMOS) logic, and Emitter Coupled Logic (ECL) implemented with silicon bipolar junction transistors (BJTs), for ICs of comparable functionality and power consumption.
There are three types of GaAs FETs that can be used for fabricating digital integrated circuits. The most common type is the Metal Semiconductor Field Effect Transistor, or MESFET. These transistors are significantly different from the more familiar silicon (Si) Metal Oxide Semiconductor FET (MOSFET). The second most common type of GaAs transistor is the Hetero Structure Isolated Gate Field Effect Transistor, or HIGFET. Although current technology is only capable of fabricating N-channel MESFETs, most HIGFET processes are capable of fabricating both N-channel and P-channel devices. These HIGFET processes are often known as C-HIGFET or CGaAs fabrication processes because of the availability of complementary transistors. The third type of GaAs FET that can be used for implementing logic circuits is the junction FET, or JFET. This transistor should not be confused with the Si JFET. The Si JFET is very common in low-frequency analog integrated circuits. However, GaAs JFETs have never had any significant advantages over other, more established technologies.